The “tension” feature is a knob that lets you change how hard you have to push to tilt the device. So, a tension tilt feature lets you choose how hard it is to lean back in a chair and how far you want to lean back.
Manufacturers may sometimes separate the functions for tilting and adjusting the tension of the tilt. We think that these office chair controls are inextricably linked, and the fact that they can’t work without each other means that they should be thought of as a single function. The tilt tension and tilt of most task, computer, and executive chairs can be changed. Tilt lets the user rock or lean back as far as the chair’s mechanism or control plate will let them. A large, spherical knob under the seat and toward the front of the chair can be turned to change the tilt tension. (Some chair makers have moved the tilt tension control to a different lever or dial on the chair, but most chairs still have it in the front.) You can change how hard it is to rock or recline the chair backward by turning the tilt tension knob. Most of the time, you have to turn the tilt tension knob several times before you can noticeably loosen or tighten the tension.
By turning this knob, you can change how tight the chair is. When the knob is turned down, it changes the tension, which makes the office chair shake. If you want it to stay in a certain position, like slightly reclined, you can tighten the tension by turning the knob in the opposite direction.
Every chair for sitting should have a knob that lets you change the angle. If you plan to sit for a long time, this knob is very important. The main purpose of these knobs is to help you find a comfortable sitting position where you can lean on your backrest and work without being too uncomfortable.
This article from FittingChairs shows how the tilt tension feature of an office chair works. You no longer have trouble answering the question, “What is the knob under my office chair?”
You can change the tilt tension by moving the often larger circular knob under the seat toward the front of the chair. (Some chair makers put the control for the tilt tension on a different lever or dial on the chair, but most don’t.) The tilt tension setting determines how much force is needed to tilt or swing the chair back.